This invention relates to medical diagnostic systems. More particularly, this invention relates to an electronic negative feedback, noise cancellation device and method. The device and method of this invention are for use with electronic medical diagnostic apparatus which require a conductive reference connection to the body of a patient.
Bio-electric signals or voltages are obtained from patient bodies for various purposes, foremost to provide information to diagnostitians. Examples of electro-medical diagnostic techniques include the electrocardiogram (EKG), electroencephalogram (EEG), and electromyogram (EMG).
Medical diagnostic apparatus which measure bio-electric signals typically require a connection between the patient's body and a reference voltage. In the past, this connection was commonly established by grounding the human body to the circuitry of the diagnostic apparatus. The physical connection was usually provided by an electrode adhesively applied to the skin and which has a conductive lead or cable which is connected to the apparatus. In the case of an EKG connection, for example, the patient's body, usually the right leg, was connected directly to the EKG equipment ground. However, this configuration provided a potential current sink in the case of equipment malfunction or patient contact with an outside current source. Because of patient safety concerns, the practice of using a direct connection to ground is not desirable and no longer used.
One accepted EKG practice is to connect a current limiting device, such as a large resistor, between the patient and the connection to ground. This provides the necessary safety factor, but will also add noise to the system. Noise, however, can interfere with the diagnostic analysis of cardiac activity by medical personnel or by electromedical apparatus. Another accepted EKG practice is to connect the patient's right leg to a reference voltage of an isolated or floating-input amplifier. The bio-electric signals from the patient body are then transmitted to the EKG apparatus for analysis via a nonconductive signal path. Examples of such nonconductive signal paths include optical fibers or transformer isolation couplers. Although this practice adds less system noise than the connection of a resistor current limiter, an appreciable level of noise or common mode voltage remains on the body due to environmental interference and which also can interfere with the accurate measurement of sensitive low-level bio-electric signals.
Two major classifications of noise exist with respect to bio-electric signals. The first, random noise, is a relatively uniform disturbance which is present throughout an entire signal. Types of random noise include Johnson noise and shot noise. Johnson noise or thermal noise is produced by thermal agitation of charges in a conductor and is characterized by a uniform energy versus frequency distribution. Johnson noise is random in that it contains no periodic components and its future value is completely unpredictable. Shot noise is exhibited by fluctuations of current output average value resulting from random emissions of electrons. Johnson and shot noise are both white, in that they have a constant energy per unit band width that is independent of the central frequency of the band. The second noise classification, periodic noise, is caused by outside interference sources such as building wire and fluorescent lights. It is not a truly random noise in that the frequencies exhibited are multiples of the basic line frequencies (i.e., 60, 120, and 180 Hz in the U.S.). Cancellation of this externally caused periodic noise is the subject of this invention.
A current EKG connection practice, which has advantages over each of the previously described methods, is the application of a negative, "noise cancelling" feedback or correction voltage to the patient's body. This practice involves sensing voltages from the body via a plurality of monitoring electrodes. The multiple voltages obtained are processed to yield an average voltage which represents, and is indicative of, the noise level on the body. The average "noise-level" voltage is then amplified by a negative amplification factor and transmitted back to the body, typically at the right leg, by an additional electrode. The negative feedback or correction voltage forces the body potential toward zero. When the patient's body attains zero voltage, the amplifier no longer drives an appreciable current and a state of equilibrium is achieved at which noise on the body is cancelled. When used in conjunction with either of the previously described current limitation or current isolation techniques, the negative feedback system provides the dual advantages of patient safety and noise cancellation in use with the EKG apparatus. Because the negative feedback is transmitted to the right leg of the human body, this practice is commonly referred to as "right leg driving."
However, a factor that significantly limits the performance of the negative feedback system is that feedback time delays destabilize the EKG or other monitoring system. The time delays are inherent in the circuitry of these prior art feedback devices due to resistance/capacitance effects of the circuit topology, which will be further described below. The time delays result in overcompensation and cause the monitoring system to oscillate and destabilize.
Prior attempts to reduce oscillations and improve stability in negative feedback systems included decreasing the sensitivity of the system. The lowering of amplification or gain of the feedback system reduces sensitivity, so that a given noise-level voltage (error voltage) will not generate as large a feedback voltage (correction voltage) for output. The major disadvantage of this technique, however, is the loss of accurate control of the parameter that is sought to be controlled. The amount of noise remaining on the body as a result of limiting the sensitivity of the negative feedback may still constitute an appreciable level when compared to the low voltage biomedical signals that may be desired to be monitored. For example, approximately 100 microvolts (uV) of 60 Hz noise may remain on the body which is a relatively large fraction of the total EKG signal, whose peak amplitude may only be 1 or 2 millivolts (mV). Although such noise levels have been reluctantly tolerated by diagnostitians using conventional EKG apparatus, modern electrocardiographic signal analysis systems measure increasingly lower level signal characteristics which are masked by these noise levels.
Although right leg driving systems such as those previously described are known in the art, insofar as is known, no solution to the problem of minimizing feedback delay without loss of sensitivity has been proposed or developed. Accordingly, it is an object of this invention to provide a method and apparatus which minimizes the time delay of a medical, negative feedback, right leg driving system without compromising patient safety or diagnostic sensitivity.